Touch probe

ABSTRACT

A touch probe ( 10,50 ) is disclosed comprising a probe body ( 12,52 ) housing first locating elements ( 23,58 ), a stylus holder ( 16,54 ) having second locating elements ( 24,57 ) which co-operate with the first locating elements to locate the stylus holder within the probe body, and a bias ( 36,68,86 ) to urge the first and second locating elements into contact, wherein an element ( 35,50,76,90 ) is provided to damp motion between the probe body and the stylus holder. The element may slow a relative movement between the locating elements perhaps by resisting the urging of the bias or absorb energy produced by a relative movement between the probe body and stylus holder. The element may be slidably or rotatably mounted with respect to one of the probe head and stylus holder.

The present invention relates to touch probes.

Touch probes having a stylus mounted in a seat within the probe andbiased into the seat are known. When the stylus contacts a surface,through relative movement of the probe and the surface, the stylus isdeflected from its seat against the action of the bias and this movementgenerates a signal which is passed to the machine so the machine canrecord the instantaneous position of the probe/surface.

The surface may belong to a workpiece whose dimensions are beingmeasured or, to a tool which is used to shape a workpiece thus enablingadjustments, to take into account tool wear, to be made to a machinecycle.

In order to make accurate measurements, the stylus needs to be seated ina repeatable location within the probe called the neutral position. Thiscan be achieved by supporting the stylus in a kinematic seat. An exampleof such a kinematic seat is described in U.S. Pat. No. 4,153,998 inwhich, the stylus is supported by three radially extending arms spacedat 120° around the axis of the stylus. The arms are supported on vnotches produced by pairs of closely spaced balls which are fixed to theprobe. A spring biases the arms into the v notches. The balls and armsform part of a circuit which is broken when the stylus is deflected. Inan alternate measurement method, instead of using the kinematic seatitself to detect deflections, strain gauges may be located so as todetect movement from the neutral position. This is described in U.S.Pat. No. 4,462,162.

If the probe stylus is deflected and released suddenly, for exampleduring a crash of the stylus into a surface or perhaps, a manualmovement of the probe, the movement of the stylus with respect to theprobe can cause shock loading of the kinematic seat when the stylus isbiased back to the seated position by the spring. This can damagefragile parts of the measuring circuit.

According to the present invention there is provided a touch probeincluding

-   -   a probe body housing first locating elements;    -   a stylus holder having second locating elements which co-operate        with the first locating elements to locate the stylus holder        within the probe body; and    -   a bias to urge the first and second locating elements into        contact, characterised in that,    -   an element is provided to damp motion between the probe body and        the stylus holder.

Preferably, the first locating elements each comprise a pair of ballswhich provide a v shaped seat and the second locating elements eachcomprise a roller which supports the stylus on the seat.

In a preferred embodiment the element slows a relative movement betweenthe first and second locating elements.

Alternatively, the element absorbs energy produced by a relativemovement between the probe body and stylus holder. An example is anelement made from a material which exhibits hysteresis for example arubber.

The invention will now be described, by way of example, with referenceto the accompanying drawings, of which:

FIG. 1 is a cross-sectional view of a probe according to the invention,

FIG. 2 is a cross-section on the line II-II of FIG. 1, of an assembledprobe,

FIG. 3 is a cross-sectional view of a probe having an alternative insertaccording to the invention,

FIG. 4 is a cross-sectional view of an alternative probe according tothe invention,

FIG. 5 is a cross-sectional view of an insert according to theinvention.

FIG. 1 and 3 show a probe 10 having an outer housing 12, preferably madeof steel, and which has a radially inwardly directed annular flange 14.Mounted within the housing 12 is a stylus holder 16 to which a stylus(not shown) may be connected.

The stylus holder 16 protrudes from the housing 12 through an opening 20for connection of a stylus and contacting of a workpiece or tool whenrelative movement takes place between the probe and the workpiece ortool.

Six recesses 22 are formed in the flange 14 and are open in thedirection facing away from the opening 20. The recesses are arranged inpairs, and the pairs are spaced apart at 120° around the longitudinalaxis of the probe 18A. The recesses are adapted to receive balls 23, andmay be of any convenient shape for example conical or triangular,whereby when the ball is received in the recess it remains in a stableposition and is kinematically supported. The spacing of the balls withineach pair is such that they may be bridged by, and form a stable seatingfor, a roller 24 carried by the stylus holder 16.

The rollers 24 are fitted in holes 26 in the stylus holder spaced at120° around the axis 18A, and the holes are initially lined with aplastic insert 28, which may be split along its length, and into whichthe rollers are pressed.

The balls are both located and clamped into their respective recesses bymeans of an open-ended cylindrical plug 30 made out of hard plastic. Inthe annular surface at its open end, the plug has six appropriatelyspaced recesses 32, which in the assembled probe, fit over the balls.Also, at its open end, the plug 30 is formed with three elongate slots34 which act as guides for the rollers 24 as the stylus holder tilts andis moved vertically by forces acting on the end of the stylus.

The stylus holder is biased into a neutral position, when no force isapplied to the stylus, by means of a spring 36 which produces a force inthe direction of the axis 18A to urge the rollers into the seats formedbetween the balls 23.

In order to enable the probe to produce a signal when the stylus isdeflected either by tilting or vertical movement, the balls areelectrically connected in series to form a circuit which is completed bythe rollers making contact with both balls in each pair.

The balls are made from a hard electrically conducting material, forexample steel or tungsten carbide, and they must therefore be insulatedfrom the housing 12 where they make contact with the housing in therecesses 22. This is achieved using a thin flexible electricallyconductive element 40 made of two layers (see FIG. 2). The first layer42 is formed from an insulating material, and the second layer 44 ispart-annular and formed from an electrically conductive material.

When the probe is assembled, the annular thin element is positioned onthe annular flange 14 with its insulating side in contact with theconducting areas of the recesses 22. Two contact pins 46 contactopposite ends of the circuit completed by the balls, rollers and thinflexible element and, at their distal end, enable access to the circuitfrom outside the probe. The balls are received in the recesses 22 in theflange 14 and in the recesses 32 in the plug. The balls are electricallyinsulated from the flange by the thin flexible element 40. The thinflexible element 40 is deformed by the kinematically located balls.Various embodiments of this flexible element are described in EP0967455.

Referring now to FIG. 1, the plug 30 has a downwardly extending centralportion 31 which lies inside the spring 36. The central portion 31 has arecess 33 in which one end of a plastic insert 35 is slidably housed.The other end of the insert 35 is located within a recess 17 of thestylus holder 16 at the distal end of the stylus holder to that whichprotrudes from the opening 20 of the housing. Each end of the insert 35has a flange which has a substantially similar diameter to the recess inwhich it is housed. The recess 17 in the stylus holder 16 has a lip 19which prevents the insert 35 from being removed from stylus recess 17.

On a stylus deflection, the stylus holder 16 moves against the spring 36pushing the insert 35 into the recess 33 of the plug 30 (and to a lesserdegree into the stylus recess 17). When the force is removed, the spring36 urges the stylus holder 16 back into its neutral position. The insert35 resists the action of the spring 36 slowing the return of the stylusholder to its neutral position thus preventing damage, in this case, tothe thin flexible element 40.

The flanges on each end of the insert 35 may be formed integrally withthe rest of the insert or, for example from plastic skirts that areglued onto the insert. The insert may be made from any suitable plasticmaterial, such as polypropylene, which is flexible enough to slidewithin the recess during a stylus deflection but stiff and tough enoughthat the flanges do not deform excessively or tear during the return ofthe stylus holder to its neutral position. The flanges form a seal ineach recess 17, 33. If the seal is too complete then fluid would not beable to bleed around the edge of each flange thus preventing the spring36 from returning the stylus holder to its neutral position. The flangesmay thus be formed so as to provide bleed zones around theircircumference or alternatively, one or more bleed holes 37 may beprovided. The use of bleed holes is preferred as fluid flow may becontrolled with greater accuracy.

Referring now to FIG. 3 which shows a probe 10 having an alternativeinsert 50. Insert 50 is manufactured from a material which exhibitshysteresis, in this case a fluorocarbon rubber. Rubbers which have largeside chains exhibit greater losses than those with smaller side chains.The insert 50 is partially housed, at one end, within recess 33 of plug30. At the other end, is formed a nipple 52 which just contacts thestylus holder 16 when the stylus holder is in its neutral position. Thisensures that the kinematic location is substantially unaffected by thepresence of the insert 50 and that the insert 50 has maximum energyabsorbing effect when the stylus is triggered.

On a stylus (not shown) deflection, the stylus holder will tilt and/ormove in the vertical (z) direction. The insert 50 is compressed by thismovement and absorbs the energy. As insert 50 is made from a rubber, itexhibits hysteresis i.e. the compression of the rubber uses more energythan is released on subsequent relaxation of the rubber (the differenceis mainly converted to heat). This effect may be enhanced by the use ofa filler within the rubber. One suitable filler is carbon powder whichconverts some of the residual compression energy into heat as thecompression of the rubber causes friction between different carbonpowder particles. A suitable amount of carbon is between 10 and 120parts per hundred (pph) and is preferably between 20 and 100 pph. Thecompression of the rubber inset 50 absorbs energy which would haveotherwise compressed the spring 36. This means that there is lesskinetic energy when the spring returns the stylus holder 16 to itsneutral position. As the insert 50 converts a high proportion of theenergy that it has removed from the system into heat (as it is lossy),it only contributes a small proportion of energy into returning thestylus holder to its neutral position thus, the impact of the rollers 24on the balls 23 and thus on the flexible element 40 is reduced. Certainrubber materials have the extra benefit that they recover from acompression at a slow rate thus the release of this energy has aminimal, if any, contribution to the effect of releasing the spring andenabling the stylus holder to return to the neutral position. An exampleof such a rubber is Sorbothane. This not only reduces normal wear andtear on the probe parts, but can also prevent damage in an abnormalsituation such as a crash of the stylus into an object.

More than one filler or other material can be added to the rubber toimprove damping of the element.

The heat generated by the insert during deflections is not sufficient tocause any thermal distortions of the kinematic location or the probe orstylus. A heat sink, such as oil, could be used within the probe cavityto minimise such a risk and this would have the added advantage that theoil, being of higher viscosity than air, would aid the damping process.

The insert 50 could be replaced by a rubber ball. The geometry of thedownwardly extending portion 31 of plug 30 would need to be changed inorder to seat the ball correctly.

Alternatively, the insert could be manufactured from a compressiblematerial which does not exhibit appreciable hysteresis and energy isremoved from the system (the insert is made lossy) by use of a fillersuch as carbon. A suitable amount of carbon is between 10 and 120 pph ofthe compressible material and preferably between 20 and 100 pph.

The insert or ball 50 need not contact the stylus holder at all times.This would ensure that the kinematic locations were not in any way beingaffected by the damper. There could even be a gap between the insert andthe stylus holder which may correspond to the size of a normaldeflection for that particular probe, so the damping effect would thenonly be used in exceptional circumstances. This of course would reducethe effectiveness of a particular insert as it cannot absorb energyuntil it is being compressed and would also require that that insertwere securely fixed in the recess to prevent removal therefrom duringnormal use of the probe.

The person skilled in the art will appreciate that a number of differentmaterials show a hysteresis effect and that the proportion of energyabsorbed is a function of the material used as well as any additives andthe conditions of use. So, an insert can be tailored to suit particularcircumstances without use of more than known material properties.

FIG. 4 is a cross-sectional view of an alternative probe 50 having ahousing 52. A stylus holder 54 is kinematically located within the probeby three arms 56 which each have a v-shaped groove 57. Each v-shapedgroove 57 partially houses one of three balls 58 mounted in the outerring of a central boss 62. The stylus holder 54 is thus supported viathe three arms 56 on the balls 58. Three webs 64 of material of reducedthickness connect the outer ring 60 to the central boss 62. The centralboss 62 is rigidly fixed to the probe housing 52. Located on each of thewebs 64, is a strain gauge 66. The v shaped grooves of the arms 56 arebiased into contact with the balls 58 by a coil spring 68 which isretained in position within the probe housing 52 by a plastic plug 70. Adamping insert 76 is retained within the spring 68 by, at a first end, arecess 74 in a downwardly extending portion 72 of the plastic plug 70and, at the other end, by a recess 78 in the stylus holder 54.

When a stylus is deflected, the stylus holder 54 moves within the probehousing 52 against the action of the spring 68. The movement is detectedby the strain gauges 66 as any change in loading on the outer ring 60causes flexure of the thin webs of material 64. The webs of material 64are each located adjacent a ball 58 making the strain gauges moresensitive to tilting movements. The signals from the strain gauges 66are relayed back to a machine controller or machine interface via twocontact pins 79 which provide electrical connection to the straingauges.

In order to make the probe as sensitive and thus as accurate aspossible, the webs of material 64 are as thin and narrow as possible.This however can lead to damage of the strain gauges as movement of theouter ring 60 with respect to the central boss 62 is magnified as thecross sectional area of the webs 64 is reduced. The probability ofdamage is increased when either high speed measurements are taking placeor if the probe crashes into a surface.

To reduce the possibility of strain gauge damage, a damper, in the formof a plastic insert 76 is used to slow down the return movement of thestylus holder 54 to its neutral, or kinematically located position. Theinsert has a triangular profiled flange at each end which allows easymovement of the flange into its respective recess 74, 78 i.e. when therehas been a stylus deflection and the spring 68 is being compressed.However, when the deflecting force is removed and the spring biastowards the neutral position returns, the flanges of the insert 76resist this return movement resulting in a reduction in the speed andforce of the return to the neutral position by the spring. So, if therehas been a large stylus (and thus stylus holder) deflection or a highforce impact the efficiency of the spring in returning the stylus holderto the neutral position is reduced.

FIG. 5 is a cross-sectional view of a further insert 90 according to theinvention. A first end 92 of the insert 90 is slidably housed in arecess 84 within a downwardly extending portion 82 of a plastic plug 80which is use would be housed within a probe (not shown). This first end92 of the insert 90 has a flange which resists downward motion of theinsert with respect to the recess 84. A second end of the insert 90 is aball 94. The ball 94 is housed within a ball joint cavity 96 of a stylusholder 88 thus enabling rotational movement of the ball with respect tothe cavity.

In kinematic positioning of one object with respect to another asdescribed above, it is preferable that friction between the two parts ofthe kinematic joint is kept to a minimum. This means that forces that donot act along the longitudinal axis of the probe should be kept to aminimum. The use of a ball joint at one end of the insert enables smoothmotion of the stylus holder with respect to the probe in a planeperpendicular to the probe axis. This assists not only as regardsrepeatability of the kinematic joint, but also by providing lessresistance to off axis movement such as tilting of the stylus holderwithin the probe cavity thus helping to retain probe sensitivity.

The type of insert used within a probe is function of the type of probeand stylus and their characteristics. Some styli are designed to becontacted mainly in the x and y planes for example, a square or discshaped stylus. In this case, the amount of overtravel required in thez-direction within the probe is limited as stylus deflections aregenerally in the xy plane. In this situation, either an energy absorbinginsert or an insert that controls the return movement to the seatedposition can be utilised.

Overtravel is the space within the probe in which the stylus holder maymove in the z-direction before contacting a part of the inside of theprobe. For styli which are as likely to deflect in direction, such asstar or ball shaped or in high speed scanning situations, moreovertravel, to protect the probe from damage, is required. This givesmore time between a deflection being registered and the probe stoppingthan a probe which has reduced overtravel. In these circumstances, theinsert which controls the return movement is more suitable.

The kinematic locating elements described herein are preferred examples.Other formations may be used. Examples are given in ‘Mechanical Designof Laboratory Apparatus’ by H Braddick Chapman and Hall Ltd, London1960.

The probes which have been described may be used in scanning ormeasuring objects for reproduction (coordinate measuring machine CMM) ormeasurement to check accuracy of a machining process (CMM or machinetool) or tool setting to account for tool wear when machining an objector with robots. Thus, the probes may be spindle mounted and movable withrespect to an object, or machine bed mounted and fixed with respect toan object, or mounted on an arm which can be removably located withinthe working volume of the machine (for example on a lathe).

1. A touch probe comprising: a probe body housing first locatingelements; a stylus holder having second locating elements whichco-operate with the first locating elements to locate the stylus holderwithin the probe body; and a bias to urge the first and second locatingelements into contact, wherein a damping element is provided to dampmotion between the probe body and the stylus holder.
 2. A touch probeaccording to claim 1 wherein said damping element slows a relativemovement between the first and second locating elements.
 3. A touchprobe according to claim 2 wherein said damping element slows therelative movement by resisting the urging of the bias.
 4. A touch probeaccording to claim 1 wherein said damping element is slidably mountedwith respect to one of the probe head and stylus holder.
 5. A touchprobe according to claim 4 wherein said damping element is slidablymounted with respect to both the probe head and stylus holder.
 6. Atouch probe according to claim 4 wherein said damping element isrotatably mounted with respect to one of the probe head and stylusholder.
 7. A touch probe according to claim 1 wherein said dampingelement absorbs energy produced by a relative movement between the probebody and stylus holder.
 8. A touch probe according to claim 7 whereinsaid damping element is lossy.
 9. A touch probe according to claim 8wherein said damping element includes at least two materials and atleast one of which is lossy.
 10. A touch probe according to claim 9wherein the lossy material is carbon powder.
 11. A touch probe accordingto claim 10 wherein between 10 and 120 pph of carbon powder is used. 12.A touch probe according to claim 1 wherein, the first locating elementseach comprise a pair of balls which form a v-shaped seat and the secondlocating elements each comprise a roller which supports the stylusholder on the v-shaped seat.
 13. A touch probe according to claim 1wherein, the first locating elements each comprise a ball and the secondlocating elements each comprise a v shaped groove which partially housesa ball and is supported thereon.